Jack Straw, the former British foreign secretary, said his instinct was that it would be right for Britain to join the US air strikes against Islamic State (Isis) in Iraq, saying there were legal grounds to do so.
He was speaking after the release of a video apparently showing the killing of a second American journalist, Steven Sotloff, and a warning from Isis that a British hostage would be the next to die.
Referring to his own role in the Iraq invasion of 2003, Straw said:"We should learn from the past, but we should not be paralysed by the past at the same time. The simple truth is these kind of barbaric jihadists were doing this kind of atrocity before the Iraq war and they would do it irrespective of the Iraq war. This is a serious threat to civilisation."
He said the logistics and legality of action against Isis inside Syria were more complex, partly because there is unlikely to be an invitation to undertake such air strikes from Syrian president Bashar al-Assad. The west has been calling for Assad to stand aside for three years.
He also called for the Kurds to be armed.
Straw voiced rare criticism of Barack Obama as the US president prepares to attend the Nato summit in Wales on Thursday.
Straw said: "He bears a burden in respect of the past. Much as I respect him, he is very long on analysis and he is not quite as fleet as foot on being decisive. You need a balance if you are the president." He said the Nato summit provided an opportunity for some very clear leadership by the US.
He refused to criticise the Cameron government, saying it was not involved in any kneejerk response. He added it was possible that some of the hostages may have been killed some time ago.
But he said the threat to British hostages shown on Tuesday's video release "will increase generally the sensitivity within the UK to this barbarity and increase the pressure for some kind of military involvement".
Asked whether Iraq had been destabilised by the 2003 invasion, he said: "We have made errors, but the truth is that al-Qaida were committing all sorts of atrocities before 9-11.
"Has Iraq contributed to the current instability in Iraq? The answer is yes. I don't know to what extent. Would Iraq be stable if there had not been an Iraq war? Not on your life. It would probably be in the same chaos as Syria."
He denied Labour's refusal to countenance action against Assad last year had worsened the position in Syria, saying the proposed military action had no clear objectives.
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